1. Field of the Invention
The present invention relates to a measuring apparatus that measures the impedance (at least one out of a purely resistive component and a reactance component) of a measured circuit.
2. Description of the Related Art
As one example of this type of measuring apparatus, a resistance measuring apparatus (hereinafter “first measuring apparatus”) disclosed in Patent Document 1 indicated below is known. This resistance measuring apparatus includes an injection transformer that injects, into a measured network (measured circuit), a current with a second frequency that is capable of being discriminated from a current with a first frequency flowing in the measured network, a detection transformer (detection coil) that detects the above two types of current flowing in the measured network, a frequency selecting circuit that takes a second frequency component out of the output of the detection transformer, and a display means that displays the output of the frequency selecting circuit. In addition, the injection transformer includes an injection coil that is provided with an output voltage of an oscillator and injects the current with the second frequency into the measured network, and a feedback coil, and is equipped with a feedback loop that varies the voltage supplied to the injection coil so as to control the voltage induced in the feedback coil to become a constant value.
In this resistance measuring apparatus, since the injection voltage obtained by dividing the voltage induced in the feedback coil by the ratio of the number of turns in the return coil to the number of connected wires in the measured network (i.e., the number of wires to which the injection transformer is attached, in this example one wire) is also controlled to become a constant value, by detecting the voltage generated across a resistor connected to the detection transformer due to the current flowing in the detection transformer, it is possible to measure a value (i.e., measured resistance) for the resistive elements connected in the measured network based on the resistance value of the resistor connected to the detection transformer, the voltage generated in such resistor, the voltage generated in the feedback coil, and the number of turns in the injection coil and the number of turns in the detection transformer (detection coil).
As another example of a measuring apparatus, the present inventors have already proposed an impedance measuring apparatus (hereinafter “second measuring apparatus”) disclosed in Patent Document 2 indicated below. This impedance measuring apparatus includes a voltage injecting unit that injects a test AC voltage into a measured circuit, a current measuring unit that measures the AC current flowing in the measured circuit due to injection of the test AC voltage, and a processing unit that calculates the impedance of the measured circuit based on the injected test AC voltage and the measured AC current. Here, the voltage injecting unit is constructed so as to be capable of injecting a plurality of different test AC voltages with different frequencies into the measured circuit and the processing unit calculates the impedance for each frequency of the test AC voltages.
As one example, assume an example configuration where test AC voltages of two frequencies are injected into the measured circuit. When a noise current (or simply “noise”) of the same frequency as one of the two frequencies is flowing in the measured circuit, due to the noise current, there is a large drop in measurement precision for the resistance value measured when a test AC voltage of such frequency is applied. In this case, the resistance value calculated (measured) when the test AC voltage of such frequency is applied will definitely be calculated as a smaller value than the resistance value calculated (measured) when the test AC voltage of the other frequency (i.e., a frequency that differs to the frequency of the noise current) is applied. This is because the AC current measured by the current measuring unit increases by an amount corresponding to the noise current being superimposed, and as a result, the resistance value of the measured circuit is calculated as a smaller value than the actual value. Accordingly, by specifying that the resistance value with the larger of the two values is the resistance value of the measured circuit, it is possible to specify the resistance value of the frequency that is less affected by noise as the resistance value of the measured circuit.
Accordingly in this impedance measuring apparatus, even when a noise current is flowing in the measured circuit, by measuring the impedance calculated when a test AC voltage of the frequency that is less affected by the noise current was injected into the measured circuit as the final impedance of the measured circuit, it is possible to reduce the effect of the noise current. As a result, it is possible to sufficiently improve the measurement precision for impedance.
Patent Document 1
    Japanese Laid-Open Patent Publication No. 555-90862 (see Pages 1 to 4 and FIG. 2)Patent Document 2    Japanese Laid-Open Patent Publication No. 2009-216618 (see Pages 1 to 6 and FIG. 1)